1. Field of the Invention
The present invention relates to a decision-feedback type equalizer, and more particularly to a decision-feedback type equalizer which is employable in a digital radio communications demodulation unit for the purpose of effecting automatic equalization in respect of transmission channel waveform distortion that arises due to fading or amplitude fluctuation, etc.
2. Description of the Prior Art
In recent years, there have been advances in modulation systems to multivalue systems, in order to make more effective use of frequencies in digital radio communication systems. On the other hand, as the number of multivalues increases, so the effects of transmission channel distortion caused by fading, etc. become greater. If intersymbol interference, etc. caused by transmission channel distortion is to be eliminated, it is necessary to effect interference compensation by means of an equalizer.
Devices known conventionally as equalizers include a decision-feedback type equalizer using transversal filters (e.g., the disclosure of Japanese Laid-open Patent Application No. 4-35113). FIG. 1 is a block diagram of one example of this conventional decision-feedback type equalizer. As seen in this figure, this conventional decision-feedback type equalizer is constituted by an A/D converter 11, a forward equalizer (FE) 12, a backward equalizer (BE) 13, an adder 14, a decision device 15 and a subtracter 16. The forward equalizer 12 and the backward equalizer 13 are each constituted by a transversal filter.
Next, the outline of the operation of the conventional decision-feedback type equalizer shown in FIG. 1 will be described. In order to simplify the description, the case considered is one in which the modulation system is an amplitude shift keying (ASK) system with symbols of the 4 values +3, +1, -1 and -3. Further, it is assumed that the decision-feedback type equalizer has a unidimensional structure compatible with the 4-value ASK system.
A baseband signal in analog form that has been demodulated by a demodulator (not shown) is input into an input terminal 10, and is converted to a pre-equalization signal Sr in digital form by analog-digital conversion effected by the A/D converter 11 in order to allow it to be processed by the decision-feedback type equalizer, which is a digital unit. If a digital baseband signal is received from the demodulator, the A/D converter 11 can be omitted.
The pre-equalization signal Sr is input, together with an error signal Sf from the subtracter 16 which is described below, into the forward equalizer 12, where an intersymbol interference quantity that is due to "lead echo" is estimated, removal of interference is effected on the basis of this estimated quantity, and output in the form of a forward equalization signal Sfe is produced. This forward equalization signal and a backward equalization signal from the backward equalizer 13, which is described below, are added by the adder 14 to produce an equalization signal Se, and this is input into the decision device 15 and into the subtracter 16.
The decision device 15 judges which symbol has been transmitted from the modulation end by comparing the equalization signal Se and the symbols of the 4 values that can be adopted, and outputs a decision signal Sd. This decision signal Sd is input, together with the error signal Sf from the subtracter 16 which is described below, into the backward equalizer 13, where an intersymbol interference quantity due to "delay echo" is estimated, removal of interference is effected on the basis of this estimated intersymbol interference quantity, and output in the form of a backward equalization signal Sbe is produced.
The decision signal Sd is also input into the subtracter 16, which takes the difference between it and the forward equalization signal Sf to produce an error signal Sf which represents the residual of equalization by the equalizer. This error signal Sf is input both into the forward equalizer 12 and the backward equalizer 13. In this manner, the decision signal Sd produced by the decision device 15 is output to an output terminal 19.
Next, an outline description of the structure of the transversal filters which constitute the forward equalizer 12 and backward equalizer 13 will be given with reference to FIG. 2, which shows an embodiment of an equalizer constituted by a transversal filter having three taps the coefficients of which are calculated by a well-known error correction algorithm, such as the Mean Square Error (MSE) Algorithm, summarized below.
Signals that are input via an input terminal 20 have their timing matched by delay elements 21 and 22, and then are respectively supplied to a 1st tap 23, a 2nd tap 24 and a 3rd tap 25. In the case where the equalizer is the forward equalizer 12, the 3rd tap 25 is the main tap.
In the 1st tap 23, the tap input and an error signal Sf that is input from a terminal 30 are multiplied by a 1st multiplier 26a, and a correlation value for the two input signals is found. Correlation values thus obtained are integrated by an integrator 27a, and a tap coefficient, which is the temporal average of the correlation values is determined. Then, a tap output is determined by multiplying this tap coefficient and the tap input by means of a 2nd multiplier 28a. This tap output represents the signal component that leaks from the main tap into the 1st tap 23.
Similarly, the 2nd tap 24 and the 3rd tap 25 consist of respective 1st multipliers 26b and 26c, integrators 27b and 27c and 2nd multipliers 28b and 28c, and the signal components that leak from the main tap into the 2nd tap 24 and 3rd tap 25 can be taken out by the 2nd multipliers 28b and 28c. The tap outputs of the 1st-3rd taps 23-25 are each supplied to an adder 29, by which the sum total of the signal components that have leaked from the main tap into the various taps is taken out. In the case of the forward equalizer 12, the output signal of this adder 29 constitutes a forward equalization signal Sfe in which the forward intersymbol interference component is removed from the pre-equalization signal, while in the case of the backward equalizer 13, it constitutes a backward equalization signal Sbe whose sign is opposite to that of the backward intersymbol interference component and, in either case, this adder 29 output signal is output from an output terminal 31. Since the invention pertains to backward equalization, for simplicity's sake, forward equalization is not discussed further, and may be assumed to add nothing to the signal, thus, Sr=Sfe.
FIG. 3, Table 1 and 2 models will be considered in order to explain the operation of the above described equalizer. FIG. 3 is a model representation of the manner in which intersymbol interference arises in a transmission channel between a transmitter and a receiver. After going via a delay element 35 and being multiplied by the correlation value [0.5] by a multiplier 36 in a backward tap 34, a transmitted symbol St which has been input from a terminal 33 is taken to be backward intersymbol interference U. This is added to the transmitted symbol St by an adder 37, and further addition of noise N is made by an adder 38, so giving a received signal Sr, which is output to a terminal 39. It is noted that noise N is not considered in the present situation.
Table 1 shows intersymbol interference U when transmitted symbols St are transmitted through the interference addition model of FIG. 3 and shows received symbols Sr, which are the sum of transmitted symbols and backward intersymbol interference U.
TABLE 1 ______________________________________ Received signals in transmission channel modem (without noise) 1 2 3 4 5 ______________________________________ Transmitted symbol (St) +3 +3 -1 +1 Intersymbol interference (U) +1.5 +1.5 -0.5 +0.5 Received symbol (Sr) +3 +4.5 +0.5 +0.5 +0.5 ______________________________________
FIG. 4 shows a model of a conventional decision-feedback type backward equalizer operation. In a backward tap 42, a decision signal Sd which has been subjected to timewise adjustment by the delay element 21 is multiplied by a tap coefficient [-0.5] by the multiplier 28 to produce a backward equalization signal Sbe. The adder 14 adds this backward equalization signal Sbe and a pre-equalization signal Sr input via a terminal 41, and so produces an equalization signal Se, which it outputs to the decision device 15. The decision device 15 judges this equalization signal Se and outputs a decision signal Sd to an output terminal 43.
Table 2 shows the values of backward equalization signals Sbe and decision signals Sd when the received signals Sr of FIG. 3 are input as pre-equalization signals Sr.
TABLE 2 ______________________________________ Decision signals in a conventional decision-feedback equalizer model (without noise) 1 2 3 4 5 ______________________________________ Pre-equalization signal (Sr) +3 +4.5 +0.5 +0.5 Backward equalization -1.5 -1.5 +0.5 -0.5 signal (Sbe) Equalization signal (Se) +3 +3 -1 +1 Decision signal (Sd) +3 +3 -1 +1 ______________________________________
It is seen from Table 1 and Table 2 that the equalizer operation results in the production of correct equalization signals Se and decision signals Sd that coincide with transmitted signals St. It is seen, therefore, that the decision-feedback equalizer is very effective in respect of removal of intersymbol interference caused by fading that occurs in a transmission channel.
Usually, when fading occurs in a transmission channel, as well an increase in intersymbol interference due to transmission channel distortion, there is also a possibility of an increase in noise due to a fall in the electric field at reception. Although an equalizer is effective for the removal of intersymbol interference, it has no effect in the suppression of interference such as noise, etc. which occurs in an uncorrelated manner. In a decision-feedback equalizer, the effect of noise on the main signal is still greater, because of the effects of error magnification or error propagation. The effect had on the main signal by such error magnification and error propagation will be described by means of modelling.
FIG. 3, which was described earlier, will be used as a model of addition of transmission channel interference, and, this time, the effect of noise which occurs without correlation to the main signal will be taken into consideration. Table 3 notes the values of intersymbol interference U, noise N and received symbols Sr when transmitted symbols St that are the same as indicated in Table 1 are transmitted through the interference model of FIG. 3.
TABLE 3 ______________________________________ Received signals in transmission channel model (with noise) 1 2 3 4 5 ______________________________________ Transmitted symbol (St) +3 +3 -1 +1 Intersymbol interference (U) +1.5 +1.5 -0.5 +1.5 Noise (N) -1.2 Received symbol (Sr) +3 +4.5 -0.7 +0.5 +0.5 ______________________________________
As seen from Table 3, The received symbol Sr at time 3 is affected by level [-1.2] noise, and it becomes a value different from that in Table 1.
On the other hand, the conventional decision-feedback type equalizer shown in FIG. 4 performs the same operation regardless of whether noise is present or not. Consequently, the values of the backward equalization signals Sbe, equalization signals Se and decision signals Sd when received symbols Sr (ignoring any forward equalization), as shown in FIG. 3 are input as pre-equalization signals to the terminal 41 of the decision-feedback type equalizer shown in FIG. 4 are as indicated in Table 4.
TABLE 4 ______________________________________ Decision signals in model of conventional decision-feedback type equalizer (with noise) 1 2 3 4 5 ______________________________________ Pre-equalization signal (Sr) +3 +4.5 -0.7 +0.5 Backward equalization -1.5 -1.5 +1.5 -0.5 signal (Sbe) Equalization signal (Se) +3 +3 -2.2 +2 Decision signal (Sd) +3 +3 -3* +3* ______________________________________ Note: * indicates a decision error.
If attention is directed to time 3 in Table 4, it is seen that, because of the effect of the noise N, the value of the equalization signal Se becomes [-2.2] and the decision threshold value is exceeded, it is incorrectly judged that a symbol whose level is [-3] has been transmitted. Because of this incorrect decision, the error component is magnified from [-1.2] to [-2.2] (this being called `error magnification`). At time 4 in Table 4, since this decision signal Sd in which the error has been magnified is used to produce a backward equalization signal Sbe, there is a succeeding occurrence of an error in the equalization signal Se and decision signal Sd, despite the fact that no noise is present.
The phenomenon whereby an error that occurs in one symbol also spreads in chain fashion to subsequent symbols is called "error propagation", and it occurs only in decision-feedback type equalizers which use decision signals Sd as feedback. However, this error propagation occurs only if there is a large amount of intersymbol interference and the tap coefficient of the backward equalizer is sufficiently large, and it does not occur if the tap coefficient is small.
As described above, decision-feedback type equalizers which are used in circuits that are subject to the effects of both intersymbol interference and noise have the drawback that one symbol error caused by noise triggers a succession of symbol errors.